Dual-perveance gridded electron gun

ABSTRACT

A dual-perveance gridded electron gun is disclosed for selectively providing a high-perveance pulsed electron beam and a low-perveance continuous electron beam. A screen grid which projects over a peripheral portion only of an electron emissive cathode surface is disposed between the emissive surface and a control grid which extends substantially across the emissive surface. Relative potentials are applied to the cathode and to the control and screen grids such that in the higher perveance mode substantially all of the cathode emissive surface emits electrons, while in the lower perveance mode electron emission is substantially precluded from the peripheral portion of the emissive surface over which the screen grid projects.

Unite ttes atent Forbess et al.

Sept. 2, 1975 DUAL-PERVEANCE GRIDDED ELECTRON GUN Primary E.\'aminer-Maynard R. Wilbur Assistant E.mminer-J. M. Potenza [75] lnvemors' Forbess Palos Verdes Attorney, Agenl, 0r Firm-W. H. MacAllister; P. M.

Peninsula; James A. Noland, Harbor Coble City, both of Calif.

[73] Assignee: Hughes Aircraft Company, Culver [57] ABSTRACT Cit Cal'f. y l A dual-perveance grldded electron gun is d1sclosed for Flledl 1973 selectively providing a high-perveance pulsed electron [21] AppL NO: 334,325 beam and a low-perveance continuous electron beam. A screen grid whlch PI'OJGCtS over a perlpheral port1on only of an electron emissive cathode surface is dis- [52] U.S. Cl; 315/31 R; 313/452 posed between the emissive urface and a control grid [51] Int. Cl. HOIJ 29/70 which extends substantially across the emissive surl Field of Search 315/31 13 R, face. Relative potentials are applied to the cathode 313/81 82 83 R, 4524155, 439 and to the control and screen grids such that in the higher perveance mode substantially all of the cathode References Cited emissive surface emits electrons, while in the lower UNITED STATES PATENTS perveance mode electron emission is substantially pre- 2.581,243 1/1952 Dodds 315 31 R eluded r m th peripheral portion of the emissive sur- 2,888,605 5 1959 Brewcr.... 315/15 face Over whlch the Scmen grld P J 3,046,442 7/1962 Cook 315/31 R 3,377,502 4/1963 Amboss ct al. 315/30 15 Clams 6 Drawmg F'gures PATENTEU EP 2 I915 SHiU 2 OF 3 PATENTED 2|975 3,903,450

SHEET 3 [If 3 Fig. 5b.

50 LI v 0 1 (Volts) J 8O 25o L Hg. 50.

lOO

(Volts) DUAL-PERVEANCE GRIDDED ELECTRON GUN This invention relates to electron beam devices, and more particularly relates to a dual-perveance gridded electron gun especially suitable for a dual-mode traveling-wave tube which provides a well-focused electron beam at both perveance levels.

In order to achieve lighter, more efficient, and less expensive ECM systems, dual-mode traveling-wave tubes have been developed in which a single tube is de signed to operate selectively in either a low-power cw mode or a high-power pulsed mode. The power level of a traveling-wave tube is a function of both the current and voltage of the electron beam used to interact with the propagating electromagnetic waves. Hence, in order to achieve dual-mode operation, either the beam current or voltage, or both, must be selectively switched between different levels in a manner sufficiently compatible with other tube parameters such that desired operation in both modes may be obtained. For a more detailed background concerning dual-mode traveling-wave tubes, reference may be made to the article Will The Real Dual-Mode TWT, Please Pulse On, Microwaves, Oct., 1972, pages 58-63.

Typical electron guns for traveling-wave tubes include an emissive cathode, a focusing electrode, and an accelerating anode. One way of obtaining dual-mode operation of a traveling-wave tube with such a gun is to apply a switching voltage to the accelerating anode in order to vary the current of the electron beam. However, in order to change the beam current sufficiently to satisfy to th high-power and low-power requirements for typical dual-mode tubes, a wide voltage swing is required at the anode, typically around 6 Kv. Alternatively, the switching voltage may be applied to the focusing electrode. However, the focusing electrode switching voltages normally required are of substantially the same magnitude as the anode switching voltages. Power supplies capable of providing such large voltages add considerably to the size, weight and expense of the electron gun. In addition, the slow-wave interaction structure of the associated traveling-wave tube usually requires essentially the same electron velocity in both the high-power and low-power modes of operation. This is accomplished by applying a voltage between the accelerating anode of the electron gun and the slow-wave structure which determines the beam perveance, defined as the ratio (beam current)/(beam voltage), and produces an electrostatic lens in the region between the anode and the slow-wave structure.

An electron gun containing a control grid can be operated with a fixed accelerating anode voltage. A large change in beam perveance can be achieved with much smaller switching voltages when the switching voltages are applied to the control grid. However, electron guns for traveling-wave tubes are normally designed with the focusing electrode and anode configurations carefully selected so that the electric field immediately outside of the electron beam is matched to the space charge forces of the beam. If the beam perveance is changed by changing the grid voltage without correspondingly adjusting the configuration of the focusing electrode and anode, the space charge forces of the beam would not be properly matched to the external electric field at the new perveance level. This not only causes nonlaminar electron flow in which the electron trajectories cross the beam axis, but in addition the minimum diameter position of the beam is shifted axially. A poorly focused beam results, giving rise to reduced beam transmission through the associated traveling-wave tube and lower tube operating efficiency.

Accordingly, it is an object of the present invention to provide a dual-perveance electron gun in which the switching between perveance levels may be achieved with relatively small switching voltages and which provides a well-focused electron beam at both perveance levels.

It is a further object of the invention to provide an electron gun capable of generating a high-perveance pulsed electron beam and a low-perveance continuous electron beam with laminar electron flow at both perveance levels.

It is a further object of the invention to provide an electron gun for generating a high-perveance pulsed electron beam and a lowpervenace continuous electron beam in which the minimum diameter of the beam remains at essentially the same axial position for both perveance levels.

It is a still further object of the invention to provide a dual-perveance electron gun which is especially suitable for a dual-mode traveling-wave tube and which enables superior beam transmission through the slowwave structure of the tube to be achieved than with dual-perveance guns of the prior art.

It is still another object of the invention to provide a dual-perveance electron gun which is especially suitable for a dual-mode traveling-wave tube and which achieves a more constant tube power output as a function of frequency than has been obtained with the prior art for both perveance levels.

In accordance with the foregoing objects, a dualperveance electron gun according to the invention includes a cathode having an electron emissive surface, an anode spaced from the cathode along a predetermined direction, a focusing electrode disposed between the cathode and the anode, a first grid electrode disposed between the cathode and the focusing electrode, and a second grid electrode disposed between the cathode and the first grid electrode. Each grid electrode has a grid disposed along a surface substantially conforming to the electron emissive surface. The first grid extends substantially across the electron emissive surface, while the second grid projects over a peripheral portion only of the electron emissive surface. Relative potentials are applied to the cathode and to the first and second grid electrodes to selectively provide either a higher perveance electron beam in which substantially all of the electron emissive surface emits electrons or a lower perveance electron beam in which the central portion of the electron emissive surface emits electrons with a density greater than that of the peripheral portion over which the second grid projects.

Additional objects, advantages and characteristic features of the invention will become readily apparent from the following detailed description of a preferred embodiment of the invention when considered in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic longitudinal sectional view illustrating a dual-perveance electron gun in accordance with the invention and operating in a high-perveance pulsed mode;

FIG 2 is a schematic longitudinal sectional view illustrating the electron gun of FIG. 1 but operating in a low-perveance continuous mode;

FIGS. 3 and 4 are plan views illustrating the respective grid arrangements of the control grid electrode and .screen grid electrode, respectively, of the electron gun of FIGS. 1 and 2; and

FIGS. 5a and 5b are graphs showing the voltage as a function of time on the control grid electrode and screen grid electrode, respectively, for both the highperveance and low-perveance modes.

Referring with greater particularity to FIGS. 1 and 2, an electron gun according to the invention may be seen to include a tubular cathode having a concave end surface 12 which is provided with a coating of electron emissive material such as barium oxide, for example. The cathode 10 may be heated in a conventional manner by means of a filament 14 energized from a source of potential 16. An annular anode 18 having a central aperture 20 through which the generated electron beam passes is coaxially mounted with respect to the cathode 10 a preselected distance away from the electron emissive surface 12. The anode 18 is maintained at a suitable dc potential V with respect to the cathode 10 by means of a source of potential 22. An annular focusing, or beam-forming, electrode 24 having a central aperture 26 through which the generated electron beam passes is coaxially mounted with respect to the cathode l0 and anode 18. The focusing electrode 24 is disposed longitudinally between the cathode 10 and anode 18, usually closer to the anode 18. The configuration of the anode 18 and focusing electrode 24 and their respective apertures 20 and 26 are designed such that the electric field immediately outside of the generated electron beam matches the space charge forces within the beam.

A control grid electrode 28 and a screen grid electrode 30 are coaxially disposed about the electron beam path between the cathode 10 and the focusing electrode 24, the screen grid electrode 30 being disposed between the cathode 10 and the control grid electrode 28. Preferably, screen grid electrode 30 is located about midway between cathode l0 and control grid electrode 28, an exemplary spacing between cathode 10 and screen grid electrode 30 being about mils. Control grid electrode 28 has an outer annular plate-like portion 32 and an inner grid portion 34, while similarly, screen grid electrode 30 has an outer annular plate-like portion 36 and an inner grid portion 38. The grid portions 34 and 38 of the respective electrodes 28 and 30 are each disposed along a concave surface substantially conforming to the cathode emissive surface 12.

Exemplary configurations for the grids 34 and 38 in accordance with a preferred embodiment of the invention are illustrated in FIGS. 3 and 4, respectively. As shown in FIG. 3, the grid 34 of control grid electrode 28 may include an inner circular web portion 40 and an intermediate circular web portion 42 concentrically disposed about the electron beam axis. A plurality of first radial web portions 44 extend between the electron beam axis and the inner circular web portion 40, with a plurality of second radial web portions 45 extending between inner circular web portion 40 and intermediate circular web portion 42 and a plurality of third radial web portions 46 extending between intermediate circular web portion 42 and outer plate-like portion 32.

As shown in FIG. 4, grid 38 of the screen grid electrode 30 may include a circular web portion 50 and a plurality of radial web portions 48 extending between circular web portion 50 and outer plate-like portion 36. Preferably, the radial web portions 48 of the grid 38 are aligned with respective outer radial web portions 46 of the grid 34. Circular web portion 50 of grid 38 defines a central circular aperture 52 having its center coincident with the electron beam axis so that the aperture 52 is aligned with the central portion of electron emissive surface 12. The diameter of circular aperture 52 should be at least about one-half (and preferably from about 0.7 to about 0.9) of the diameter of the circular periphery of the electron emissive surface 12. Thus, the grid 38 of screen grid electrode 30 projects over an annular peripheral portion only of the electron emissive surface 12 while leaving the central portion of emissive surface 12 directly exposed to the central portion of control grid electrode 28.

An exemplary arrangement for applying suitable potentials to the control grid electrode 28 and the screen grid electrode 30 in order to selectively achieve dualperveance operation of the electron gun of the invention in both a high-perveance pulsed mode and a lowperveance continuous mode is illustrated in FIGS. 1 and 2. A source of potential 54 providing a dc bias voltage V of a sufficient value to preclude electron emission from the cathode 10 has its positive terminal connected to cathode 10. A pulse source 56 providing voltage pulses of amplitude V is connected between the negative terminal of source 54 and a first (or pulsed operation) contact terminal 58 of a switch 60. A source of potential 62 providing a dc voltage V of larger magnitude than the voltage V has its negative terminal connected to the negative terminal of source 54 and its positive terminal connected to a second (or continuous operation) contact terminal 64 of switch 60. A voltage divider, illustrated as a potentiometer 66, is connected in parallel with pulse source 56, with the potentiometer tap connected to a first (or pulsed operation) contact terminal 68 of a switch 70. Similarly, a voltage divider, illustrated as a potentiometer 72, is connected in parallel with dc source 62, with the potentiometer tap connected to a second (or continuous operation) contact terminal 74 of switch 70.

Switch has a contact arm 76 connected to screen grid electrode 30, while switch 60 has a contact arm 78 connected to control grid electrode 28. In the particular embodiment of the invention illustrated in FIGS. 1 and 2, switch contact arm 78 is also connected to focusing electrode 24 in order to apply the control grid potential to the focusing electrode 24. However, it shouuld be understood that other focusing electrode potentials, for example cathode potential, are also suitable. It is further pointed out that switches 60 and 70 are shown schematically as mechanical switches solely for explanatory purposes. While these switches may comprise relays or other electro-mechanical devices, electronic switches utilizing vacuum tubes or diodes, for example, are also suitable and would be employed when it is desired to obtain rapid switching between the high-perveance and low-perveance modes.

Specific exemplary values for the voltages V V,,, V and V,. which have been employed in a particular dualperveance electron gun constructed according to the invention are as follows:

V 6900 volts V,, volts V,, 359 volts V 250 volts.

It is pointed out, of course, that the aforementioned voltages are given solely for exemplary purposes, and the particular values to be used in a given situation would be selected according to particular gun requirements.

Using the aforementioned exemplary voltages, a dual-perveance electron gun constructed according to FIGS. 14 was operated with the following values for the voltage V, at the control grid electrode 28 and the voltage V at the screen grid electrode 30 (measured with respect to the cathode 10), the current I at control grid electrode 28, the current I at screen grid electrode 30, the current I,,,.,,,,, of the generated electron beam, and the perveance P of the electron gun as defined above.

Pulsed Mode Continuous Mode V +234 volts +125 volts V +47 volts -78 volts I 35 ma 7 ma I, 166 mu Iboum 1.38 amps 034 amps P 2.0 X 10 X The operation of the dual-perveance electron gun of FIGS. 14 will now be discussed with reference to the waveforms of FIGS. 5a and 5b. When the switches 60 and 70 are as shown in FIG. 1 with their respective contact arms 78 and 76 electrically contacting terminals 58 and 68, respectively, the electron gun will operate in the high-perveance pulsed mode. In this mode the voltage V applied to control grid electrode 28 consists of a series of voltage pulses 80 (FIG. 5a), while the voltage V,, applied to screen grid electrode 30 consists of a series of voltage pulses 82 coincident in time with the pulses 80 but of a smaller amplitude as determined by the setting of the tap on potentiometer 66. In the absence of pulses 80 and 82, the potential applied to the control grid electrode 28 and the screen grid electrode 30 is determined by the bias voltage V,, of source 54, and as was mentioned above, this voltage is sufficient to prevent the emission of an electron beam from cathode surface 12. In the presence of pulses 80 and 82, the respective potentials applied to control grid electrode 28 and screen grid electrode 30 are such that a relatively high-perveance electron beam is emitted from the cathode surface 12 and, under the influence of the electric field from focusing electrode 24 and anode 18, is caused to flow along a path indicated by dashed lines 84 of FIG. 1.

When the switches 60 and 70 are as shown in FIG. 2 with their respective contact arms 78 and 76 making electrical contact with terminals 64 and 74, respectively, the electron gun is set for operation in the lowperveance continuous mode. In this mode the voltage V applied to the control grid electrode 28 is determined by the algebraic sum of the voltages provided by the dc sources 62 and 54. Since the magnitude of the voltage V of source 62 is greater than the magnitude of voltage V,, of source 54, a net positive voltage with respect to the cathode 10 (shown at level 86 of FIG. 5a) is applied to control grid electrode 28. The voltage V, applied to the screen grid electrode 30 is determined not only by the voltages V and V, of sources 62 and 54, respectively, but also by the setting of the tap on potentiometer 72. This tap is preferably set such that the voltage V,, applied to screen grid electrode 30 (shown at level 88 of FIG. 5b) is sufficiently negative with respect to the cathode 10 to substantially preclude electron emission from the annular peripheral portion of the cathode surface 12 over which the screen grid 38 projects. A relatively low-perveance electron beam is thus emitted from cathode surface 12 and, under the influence of the electric field from focusing electrode 24 and anode 18, is caused to flow along a path shown by dashed lines 90 of FIG. 2.

By changing the cross-section of the emitted electron beam when the beam is switched between the highperveance and low-perveance levels in the manner described above, the space charge forces of the beam are well matched with the electric field immediately outside of the beam at both perveance levels. Substantially laminar electron flow is achieved, and in addition, the axial position at which the minimum beam diameter occurs remains substantially unchanged. A well-focused beam results at both perveance levels. This enables increased beam transmission through the associated traveling-wave tube and higher tube operating efficiency to be attained.

A further advantage of the electron gun of the invention is that it achieves a more constant power output as a function of frequency for the associated travelingwave tube than has been obtained with prior art dualperveance electron guns. Specifically, both a dualperveance gridded electron gun constructed according to FIGS. 14 using the aforementioned specific parameter values and a dual-perveance gridded electron gun according to the prior art wherein the gun perveance was changed by varying the voltage applied to the control grid were used to operate otherwise identical octave bandwidth, S-band helix traveling-wave tubes with their focusing arrangements optimized for the lowpower cw mode. It was found that a dual-perveance electron gun according to the present invention reduced the amount of beam power intercepted by the helix slow-wave structure by a factor of 2.5. Moreover, in the low-power cw mode the variation in the tube output power as a function of frequency across the octave bandwidth of the tube was reduced from 1.6 db to 1.1 db, and in the high-power pulsed mode the output power variation as a function of frequency across the octave bandwidth was reduced from 3.3 db to only 0.5 db.

Although the invention has been shown and described with reference to a particular embodiment, various changes and modifications may occur to a person skilled in the art to which the invention pertains. For example, a shadow grid having the same configuration as grid 34 and maintained at the potential of cathode 10 may be interposed between cathode l0 and grid 38 to minimize electron interception on grids 34 and 38. Modifications of this type which are obvious to one skilled in the art are deemed to lie within the spirit, scope and contemplation of the invention.

What is claimed is:

l. A dual-perveance electron gun comprising:

a cathode having an electron emissive surface;

an anode spaced from said cathode along a predetermined direction;

a focusing electrode disposed between said cathode and said anode;

a first grid electrode disposed between said cathode and said focusing electrode, said first grid electrode having a grid disposed along a surface substantially conforming to said electron emissive surface and extending substantially across said electron emissive surface;

a second grid electrode disposed between said first grid electrode and said cathode, said second grid electrode having a grid disposed along a surface substantially conforming to said electron emissive surface and projecting over a peripheral portion only of said electron emissive surface; and

means for applying relative potentials to said cathode and said first and second grid electrodes to selectively provide either a higher perveance electron beam in which substantially all of said electron emissive surface emits electrons or a lower perveance electron beam in which the central portion of said electron emissive surface emits electrons with a density greater than that of said peripheral portion.

2. A dual-perveance electron gun according to claim 1 wherein said focusing electrode is electrically connected to said first grid electrode.

3. A dual-perveance electron gun according to claim 1 wherein said relative potentials are such as to substantially preclude electron emission from said peripheral portion of said electron emissive surface for saidlower perveance electron beam.

4. A dual-perveance electron gun according to claim 1 wherein said means for applying relative potentials includes:

first means for applying voltage pulses to said first and second grid electrodes such that electron emission from said electron emissive surface is substantially precluded in the absence of said voltage pulses and said higher perveance electron beam is provided in the presence of said voltage pulses; second means for applying relative dc potentials to said first and second grid electrodes and said cathode such that said lower perveance electron beam is provided; and switch means for selectively operatively coupling either said first means or said second means to said first and second grid electrodes and said cathode. 5. A dual-perveance electron gun according to claim 4 wherein said voltage pulses applied to said first and second grid electrodes by said first means are such that during the presence of said voltage pulses said second grid electrode is at a positive potential with respect to said cathode and said first grid electrode is at a higher positive potential with respect to said cathode.

6. A dual-perveance electron gun according to claim 4 wherein said relative dc potentials applied to said first and second grid electrodes and said cathode by said second means are such that said first grid electrode is biased positively with respect to said cathode and said second grid electrode is biased negatively with respect to said cathode.

7. A dual-perveance electron gun according to claim 4 wherein said relative dc potentials are such as to substantially preclude electron emission from said peripheral portion of said electron emissive surface for said lower perveance electron beam.

8. A dual-perveance electron gun comprising: a cathode having a concave electron emissive surface defining a figure of revolution about a predetermined axis;

An annular anode spaced from and facing said electron emissive surface and coaxially disposed about said predetermined axis;

an annular focusing electrode coaxially disposed about said predetermined axis between said cathode and said anode;

a first grid electrode disposed between said cathode and said focusing electrode, said first grid electrode having a grid disposed along a concave surface substantially conforming to said electron emissive surface and defining a figure of revolution about said predetermined axis, the grid of said first grid electrode extending substantially across said electron emissive surface;

a second grid electrode disposed between said first grid electrode and said cathode, said second grid electrode having a grid disposed along a concave surface substantially conforming to said electron emissive surface and defining a figure of revolution about said predetermined axis, the grid of said second grid electrode projecting over an annular peripheral portion only of said electron emissive surface and defining a circular central aperture having a diameter not less than about one-half of the diameter of the circular periphery of said electron emissive surface, said central aperture being aligned with the central portion of said electron emissive surface; and

means for applying relative potentials to said cathode and said first and second grid electrodes to selectively provide either a higher perveance electron beam in which substantially all of said electron emissive surface emits electrons or a lower perveance electron beam in which said central portion of said electron emissive surface emits electrons with a density greater than that of said peripheral portion.

9. A dual-perveance electron gun according to claim 8 wherein said focusing electrode is electrically connected to said first grid electrode.

10. A dual-perveance electron gun according to claim 8 wherein said relative potentials are such as to substantially preclude electron emission from said peripheral portion of said electron emissive surface for said lower perveance electron beam.

11. A dual-perveance electron gun according to claim 8 wherein said means for applying relative potentials includes:

first means for applying voltage pulses to said first and second grid electrodes such that electron emission from said electron emissive surface is substantially precluded in the absence of said voltage pulses and said higher perveance electron beam is provided in the presence of said voltage pulses;

second means for applying relative dc potentials to said first and second grid electrodes and said cathode such that said lower perveance electron beam is provided; and

switch means for selectively operatively coupling either said first means or said second means to said first and second grid electrodes and said cathode.

12. A dual-perveance electron gun according to claim 11 wherein said voltage pulses applied to said first and second grid electrodes by said first means are such that during the presence of said voltage pulses said second grid electrode is at a positive potential with respect to said cathode and said first grid electrode is at a higher positive potential with respect to said cathode.

13. A dual-perveanee electron gun according to claim 11 wherein said relative dc potentials applied to said first and second grid electrodes and said cathode by said second means are such that said first grid electrode is biased positively with respect to said cathode and said second grid electrode is biased negatively with respect to said cathode.

14. A dual-perveanee electron gun according to claim 1 1 wherein said relative dc potentials are such as to substantially preclude electron emission from said peripheral portion of said electron emissive surface for said lower perveanee electron beam.

15. A dual-perveanee electron gun according to claim 8 wherein each of said first and second grid electrodes has a peripheral portion defining substantially a circular aperture;

the grid of said first grid electrode having first and second circular web portions of differing diameters the grid of said second grid electrode having a circular web portion defining said central aperture and a plurality of radial web portions extending between said web portion defining said central aperture and the peripheral portion of the grid of said second grid electrode substantially in alignment with respective ones of said third radial web por- 

1. A dual-perveance electron gun comprising: a cathode having an electron emissive surface; an anode spaced from said cathode along a predetermined direction; a focusing electrode disposed between said cathode and said anode; a first grid electrode disposed between said cathode and said focusing electrode, said first grid electrode having a grid disposed along a surface substantially conforming to said electron emissive surface and extending substantially across said electron emissive surface; a second grid electrode disposed between said first grid electrode and said cathode, said second grid electrode having a grid disposed along a surface substantially conforming to said electron emissive surface and projecting over a peripheral portion only of said electron emissive surface; and means for applying relative potentials to said cathode and said first and second grid electrodes to selectively provide either a higher perveance electron beam in which substantially all of said electron emissive surface emits electrons or a lower perveance electron beam in which the central portion of said electron emissive surface emits electrons with a density greater than that of said peripheral portion.
 2. A dual-perveance electron gun according to claim 1 wherein said focusing electrode is electrically connected to said first grid electrode.
 3. A dual-perveance electron gun according to claim 1 wherein said relative potentials are such as to substantially preclude electron emission from said peripheral portion of said electron emissive surface for said lower perveance electron beam.
 4. A dual-perveance electron gun according to claim 1 wherein said means for applying relative potentials includes: first means for applying voltage pulses to said first and second grid electrodes such that electron emission from said electron emissive surface is substantially precluded in the absence of said voltage pulses and said higher perveance electron beam is provided in the presence of said voltage pulses; second means for applying relative dc potentials to said first and second grid electrodes and said cathode such that said lower perveance electron bEam is provided; and switch means for selectively operatively coupling either said first means or said second means to said first and second grid electrodes and said cathode.
 5. A dual-perveance electron gun according to claim 4 wherein said voltage pulses applied to said first and second grid electrodes by said first means are such that during the presence of said voltage pulses said second grid electrode is at a positive potential with respect to said cathode and said first grid electrode is at a higher positive potential with respect to said cathode.
 6. A dual-perveance electron gun according to claim 4 wherein said relative dc potentials applied to said first and second grid electrodes and said cathode by said second means are such that said first grid electrode is biased positively with respect to said cathode and said second grid electrode is biased negatively with respect to said cathode.
 7. A dual-perveance electron gun according to claim 4 wherein said relative dc potentials are such as to substantially preclude electron emission from said peripheral portion of said electron emissive surface for said lower perveance electron beam.
 8. A dual-perveance electron gun comprising: a cathode having a concave electron emissive surface defining a figure of revolution about a predetermined axis; An annular anode spaced from and facing said electron emissive surface and coaxially disposed about said predetermined axis; an annular focusing electrode coaxially disposed about said predetermined axis between said cathode and said anode; a first grid electrode disposed between said cathode and said focusing electrode, said first grid electrode having a grid disposed along a concave surface substantially conforming to said electron emissive surface and defining a figure of revolution about said predetermined axis, the grid of said first grid electrode extending substantially across said electron emissive surface; a second grid electrode disposed between said first grid electrode and said cathode, said second grid electrode having a grid disposed along a concave surface substantially conforming to said electron emissive surface and defining a figure of revolution about said predetermined axis, the grid of said second grid electrode projecting over an annular peripheral portion only of said electron emissive surface and defining a circular central aperture having a diameter not less than about one-half of the diameter of the circular periphery of said electron emissive surface, said central aperture being aligned with the central portion of said electron emissive surface; and means for applying relative potentials to said cathode and said first and second grid electrodes to selectively provide either a higher perveance electron beam in which substantially all of said electron emissive surface emits electrons or a lower perveance electron beam in which said central portion of said electron emissive surface emits electrons with a density greater than that of said peripheral portion.
 9. A dual-perveance electron gun according to claim 8 wherein said focusing electrode is electrically connected to said first grid electrode.
 10. A dual-perveance electron gun according to claim 8 wherein said relative potentials are such as to substantially preclude electron emission from said peripheral portion of said electron emissive surface for said lower perveance electron beam.
 11. A dual-perveance electron gun according to claim 8 wherein said means for applying relative potentials includes: first means for applying voltage pulses to said first and second grid electrodes such that electron emission from said electron emissive surface is substantially precluded in the absence of said voltage pulses and said higher perveance electron beam is provided in the presence of said voltage pulses; second means for applying relative dc potentials to said first and second grid electrodes and said cathode such that said lower perveance elEctron beam is provided; and switch means for selectively operatively coupling either said first means or said second means to said first and second grid electrodes and said cathode.
 12. A dual-perveance electron gun according to claim 11 wherein said voltage pulses applied to said first and second grid electrodes by said first means are such that during the presence of said voltage pulses said second grid electrode is at a positive potential with respect to said cathode and said first grid electrode is at a higher positive potential with respect to said cathode.
 13. A dual-perveance electron gun according to claim 11 wherein said relative dc potentials applied to said first and second grid electrodes and said cathode by said second means are such that said first grid electrode is biased positively with respect to said cathode and said second grid electrode is biased negatively with respect to said cathode.
 14. A dual-perveance electron gun according to claim 11 wherein said relative dc potentials are such as to substantially preclude electron emission from said peripheral portion of said electron emissive surface for said lower perveance electron beam.
 15. A dual-perveance electron gun according to claim 8 wherein each of said first and second grid electrodes has a peripheral portion defining substantially a circular aperture; the grid of said first grid electrode having first and second circular web portions of differing diameters concentrically disposed about said predetermined axis, a plurality of first radial web portions extending between said predetermined axis and said first circular web portion, a plurality of second radial web portions extending between said first and second circular web portions, and a plurality of third radial web portions extending between said second circular web portion and the peripheral portion of the grid of said first grid electrode; and the grid of said second grid electrode having a circular web portion defining said central aperture and a plurality of radial web portions extending between said web portion defining said central aperture and the peripheral portion of the grid of said second grid electrode substantially in alignment with respective ones of said third radial web portions. 